The objectives of the proposed research are to define the role of inner medullary blood flow in the regulation o renal salt excretion and to delineate the relationship between cortical blood flow distribution and inner medullary plasma flow. In the present proposal evidence is presented suggesting that a marked reduction in papillary plasma flow with increased solute content is involved in the salt retention of a salt-retaining model, the dog with chronic constriction of the thoracic inferior vena cava with ascites (caval dog), and is associated with no change in the cortical blood flow distribution. The methods will include chronic sodium balance studies with acute experiments in which glomerular filtration rate, sodium excretion, cardiac output, renal blood flow, cortical blood flow distribution using radio-labeled microspheres, papillary plasma flow using the 125I-albumin accumulation technique and papillary solute content will be determined. These basic protocols will be utilized to evaluate a) two other models of salt retention: dogs with salt depletion and with aorto-caval fistula; b) normal dogs under renal vasodilatation (acetylcholine and bradykinin); normal dogs under diuretics (furosemide and chlorothiazide); d) six perturbations to induce natriuresis in caval dogs: acute release of caval constriction, renal vasodilatation followed by norepinephrine infusion, administration of furosemide, administration of angiotensin II, and production of papillary necrosis by bromoethylamine hydrobromide administration. Correlations will be evaluated for cumulative sodium balance, sodium excretion, papillary plasma flow, papillary solute content, and cortical distribution of microspheres.